HTML Canvas：基本操作

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• HTML5 CANVAS ELEMENTHtml5 canvas element

  <canvas id="myCanvas" width="500" height="300">

  Html5 canvas element with fallback content

  <canvas id="myCanvas" width="500" height="300">  your browser doesn‘t support canvas!</canvas>

  2d context

  var context = canvas.getContext(‘2d‘);

  Webgl context (3d)

  var context = canvas.getContext(‘webgl‘);

• SHAPESDraw rectangle

  context.rect(x, y, width, height);context.fill();context.stroke();

  Fill rectangle shorthand

  context.fillRect(x, y, width, height);

  Stroke rectangle shorthand

  context.strokeRect(x, y, width, height);

  Draw circle

  context.arc(x, y, radius, 0, Math.PI * 2);context.fill();context.stroke();

• STYLESFill

  context.fillStyle = ‘red‘;context.fill();

  Stroke

  context.strokeStyle = ‘red‘;context.stroke();

  Linear gradient

  var grd = context.createLinearGradient(x1, y1, x2, y2);grd.addColorStop(0, ‘red‘);   grd.addColorStop(1, ‘blue‘);context.fillStyle = grd;context.fill();

  Radial gradient

  var grd = context.createRadialGradient(x1, y1, radius1, x2, y2, radius2);grd.addColorStop(0, ‘red‘);grd.addColorStop(1, ‘blue‘);context.fillStyle = grd;context.fill();

  Pattern

  var imageObj = new Image();imageObj.onload = function() {  var pattern = context.createPattern(imageObj, ‘repeat‘);  context.fillStyle = pattern;  context.fill();};imageObj.src = ‘path/to/my/image.jpg‘;

  Line Join

  context.lineJoin = ‘miter|round|bevel‘;

  Line Cap

  context.lineCap = ‘butt|round|square‘;

  Shadow

  context.shadowColor = ‘black‘;context.shadowBlur = 20;context.shadowOffsetX = 10;context.shadowOffsetY = 10;

  Alpha (Opacity)

  context.globalAlpha = 0.5; // between 0 and 1

• COLOR FORMATSString

  context.fillStyle = ‘red‘;

  Hex Long

  context.fillStyle = ‘#ff0000‘;

  Hex Short

  context.fillStyle = ‘#f00‘;

  RGB

  context.fillStyle = ‘rgb(255,0,0)‘;

  RGBA

  context.fillStyle = ‘rgba(255,0,0,1)‘;

• PATHSBegin Path

  context.beginPath();

  Line

  context.lineTo(x, y);

  Arc

  context.arc(x, y, radius, startAngle, endAngle, counterClockwise);

  Quadratic curve

  context.quadraticCurveTo(cx, cy, x, y);

  Bezier curve

  context.bezierCurveTo(cx1, cy1, cx2, cy2, x, y);

  Close Path

  context.closePath();

• IMAGESDraw Image with default size

  var imageObj = new Image();imageObj.onload = function() {  context.drawImage(imageObj, x, y);};imageObj.src = ‘path/to/my/image.jpg‘;

  Draw image and set size

  var imageObj = new Image();imageObj.onload = function() {  context.drawImage(imageObj, x, y, width, height);};imageObj.src = ‘path/to/my/image.jpg‘;

  Crop image

  var imageObj = new Image();imageObj.onload = function() {  context.drawImage(imageObj, sx, sy, sw, sh, dx, dy, dw, dh);};imageObj.src = ‘path/to/my/image.jpg‘;

• TEXTFill Text

  context.font = ‘40px Arial‘;context.fillStyle = ‘red‘;context.fillText(‘Hello World!‘, x, y);

  Stroke Text

  context.font = ‘40pt Arial‘;context.strokeStyle = ‘red‘;context.strokeText(‘Hello World!‘, x, y);

  Bold Text

  context.font = ‘bold 40px Arial‘;

  Italic Text

  context.font = ‘italic 40px Arial‘;

  Text Align

  context.textAlign = ‘start|end|left|center|right‘;

  Text Baseline

  context.textBaseline = ‘top|hanging|middle|alphabetic|ideographic|bottom‘;

  Get Text Width

  var width = context.measureText(‘Hello world‘).width;

• TRANSFORMSTranslate

  context.translate(x, y);

  Scale

  context.scale(x, y);

  Rotate

  context.rotate(radians);

  Flip Horizontally

  context.scale(-1, 1);

  Flip Vertically

  context.scale(1, -1);

  Custom Transform

  context.transform(a, b, c, d ,e, f);

  Set Transform

  context.setTransform(a, b, c, d ,e, f);

  Shear

  context.transform(1, sy, sx, 1, 0, 0);

  Reset

  context.setTransform(1, 0, 0, 1, 0, 0);

• STATE STACKPush State onto Stack

  context.save();

  Pop State off of Stack

  context.restore();

• CLIPPINGClip

  // draw path herecontext.clip();

• IMAGE DATAGet Image Data

  var imageData = context.getImageData(x, y, width, height);var data = imageData.data;

  Loop Through Image Pixels

  var imageData = context.getImageData(x, y, width, height);var data = imageData.data;var len = data.length;var i, red, green, blue, alpha;for(i = 0; i < len; i += 4) {  red = data[i];  green = data[i + 1];  blue = data[i + 2];  alpha = data[i + 3];}

  Loop Through Image Pixels With Coordinates

  var imageData = context.getImageData(x, y, width, height);var data = imageData.data;var x, y, red, green, blue, alpha;for(y = 0; y < imageHeight; y++) {  for(x = 0; x < imageWidth; x++) {    red = data[((imageWidth * y) + x) * 4];    green = data[((imageWidth * y) + x) * 4 + 1];    blue = data[((imageWidth * y) + x) * 4 + 2];    alpha = data[((imageWidth * y) + x) * 4 + 3];  }}

  Set Image Data

  context.putImageData(imageData, x, y);

• DATA URLSGet Data URL

  var dataURL = canvas.toDataURL();

  Render Canvas with Data URL

  var imageObj = new Image();imageObj.onload = function() {  context.drawImage(imageObj, 0, 0);};imageObj.src = dataURL;

• COMPOSITESComposite Operations

  context.globalCompositeOperation = ‘source-atop|source-in|source-out|source-over|destination-atop|destination-in|destination-out|destination-over|lighter|xor|copy‘;

HTML Canvas：基本操作